1. Field of the Invention
This invention relates to an electrical storage device, a vehicle powered by such an electrical storage device, and a method and apparatus for making such an electrical storage device. More particularly, the invention relates to a storage device made by winding layers of materials, a vehicle powered by such a storage cell, and a method and apparatus for making such a storage device.
2. Description of the Related Art
In recent years, with increasing emphasis on energy conservation and pollution control, various types of electrically powered vehicles have been proposed. Generally, these vehicles are powered by some sort of battery. Due to the cost of the battery or batteries needed to power a vehicle, rechargeable batteries are generally utilized in such vehicles. Such rechargeable batteries may be charged by solar energy using photoelectric cells which convert light energy into electrical energy. However, solar powered batteries are very expensive and are not efficient enough for a practical electric vehicle. Further, solar cells require daylight to operate, thereby limiting the distance which the vehicle could travel in one trip, and limiting travel at night.
Other rechargeable batteries are charged by direct connection to a source of electrical energy. In an electrically powered vehicle where the batteries are charged through electricity rather than through solar energy, the storage capacity of the battery has been a limiting factor affecting the speed at which the vehicle can travel, and the distance that the vehicle can travel before having to recharge its batteries. Different types of rechargeable batteries have been proposed for use in such vehicles.
For example, it has been proposed to use a plurality of prismatic batteries similar to typical automobile batteries. Prismatic automobile batteries usually include a number of separate cells or chambers, each of which contains multiple positive electrodes made of lead oxide, multiple negative electrodes made of lead, and an electrolyte solution such as sulfuric acid. The cells are connected in series with two terminals provided to connect the battery to an external circuit. When the terminals are connected to the circuit, a chemical reaction occurs in the cells. In this reaction, the negative electrodes are oxidized (producing electrons) and the positive electrodes are reduced. Electrons flow from the negative electrode to the positive electrode through the external circuit to do work discharging the battery. The battery can be recharged by passing direct current through it in the direction opposite to the direction of current flow during discharge thereby reversing the chemical reaction. Thus, during recharging, the negative electrodes are reduced and the positive electrodes are oxidized.
For a given reaction direction, the oxidized electrode is typically called an "anode" and the reduced electrode is called a "cathode." For simplicity herein, and consistent with common usage, the electrode reduced during discharge will be called the "cathode," and the electrode oxidized during discharge will be call the "anode", even though during recharge the electrodes' roles are reversed.
Many problems exist, however, in creating a prismatic battery suitable for powering a vehicle. For example, prismatic batteries are very bulky and heavy and, as such, reduce the distance the vehicle can travel on a charge and the top speed that the vehicle can achieve. Further, these batteries generally require a long period of time, on the order of several hours, to recharge. Therefore, prismatic battery powered vehicles are not practical for traveling between locations more distant than the vehicle can travel on a single charge. Moreover, if a prismatic, lead-acid battery powered vehicle were to be involved in a serious accident, its passengers and the environment would be at risk of harm from contact with acid spilled from broken batteries.
Another type of generally known rechargeable battery is a wound or coil battery. In wound batteries, the two electrode plates made of long strips of material are wound together into a coil, with one or more pieces of separator material wound between the coils (to prevent shorting), and an electrolyte placed in the coil to allow current flow. Tabs are welded to each of the electrodes and connected to respective positive and negative terminals. As for prismatic batteries, when the terminals are connected to an external circuit, a chemical reaction occurs causing current to flow. The battery is then recharged by driving direct current through the battery in the reverse direction to recharge the battery.
One type of wound battery is a nickel-cadmium battery, which is the commonly available rechargeable battery used for consumer products. A nickel-cadmium battery includes a positive electrode made of nickel hydroxide, a negative plate made of cadmium, and an aqueous alkaline solution, such as potassium hydroxide, as the electrolyte.
The operating parameters of a wound battery are a function of the dimensions of its electrodes. For example, the capacity of a wound battery to store charge, and the corresponding ability of the battery to produce current for a period of time (often measured in amp-hours), is dependent upon the mass of active material making up the electrode plates. The available current of the battery is roughly proportional to the amount of overlapping surface area of the electrodes within the coil.
It has therefore been proposed to form wound cells having thin electrode plates to increase the amount of overlapping surface area within a battery of a given diameter to increase available current. However, attempts to make wound batteries with very thin plates proved unsuccessful for a number of reasons.
For example, the strength of the electrodes is roughly proportional to their thickness. Therefore, thinner than usual electrodes are liable to undesirably bend or break during manufacture of the battery.
Further, with thinner than usual electrodes, the portion of the volume of the wound battery which is taken up by the separator will necessarily be greater. This result occurs because more windings of thinner electrodes will fit within the battery, and the electrodes will therefore be longer. However, because the electrodes are longer, the separator must also be longer. In that the separator thickness is not a function of the electrode thickness, in a wound battery with thinner electrodes, the separator occupies more volume than if the electrodes were not thinner. Thus, in a battery with thinner than usual electrodes, the mass of the electrodes within the battery, and consequently the capacity of the battery is undesirably reduced.
It has also been proposed to construct wound batteries having longer than usual electrodes which are as thick as conventional electrodes. This type of battery would thus have a diameter greater than that of a conventional battery. This type of battery would have a higher capacity than a conventional battery due to longer (more massive) electrodes and would have a higher current output than a conventional battery due to increased overlapping surface between the electrodes. However, a number of problems exist in the manufacture and use of such a wound battery.
For example, long electrodes are more likely to be bent, broken or non-uniformly wound during the battery manufacturing process which could slow their manufacture, or create gaps or shorts in the battery rendering it useless. Long electrodes also require more precise control of the feeding of the electrodes and separator during the winding process. If either of the electrodes are even slightly misaligned, winding the electrodes repeatedly as is required to form a winding will cause the battery to fail to achieve the desired cylindrical shape. Instead, the winding will be slightly conical and, with each turn, the electrodes will be undesirably offset from each other by a greater distance. Such a misaligned winding generally cannot be used and probably will not function due to contact between the plates. Similarly, if the separator is not properly aligned or tears during winding, the electrodes will contact each other thereby shorting the winding.
Moreover, with long electrodes, more tabs must be welded to each electrode to uniformly draw current from the electrode. Welding the additional tabs is labor intensive making manufacturing difficult and making the winding less uniform due to the increased frequency of the tabs projecting into the layers at more points in the battery.
Further, long windings with large diameters are necessarily heavier than their smaller counterparts making uniform support of the winding during the step of manufacturing difficult. Since the electrodes are usually wound about an arbor, heavier windings can cause the arbor to bend or break under the weight of the wound electrodes thereby ruining the battery.
In that a single wound battery often is too small to provide the voltage, capacity, or current required for a given application, groups of wound batteries have been connected in some fashion to provide increased output. Connecting batteries in series provides a total voltage equal to the sum of the individual voltages across each battery. Connecting batteries in parallel provides a total capacity and current equal to the sum of the individual capacities and, respectively, of the individual currents, of each battery. Connecting groups of batteries in series, and then connecting the groups in parallel provides increased voltage, capacity, and current. However, such combinations of individual batteries suffer from several drawbacks.
For example, a plurality of individual rechargeable batteries obviously requires more space and weighs more than a single battery. While wound batteries are generally volumetrically more efficient than prismatic batteries (that is, wound batteries can provide more capacity per unit of volume), use of a plurality of conventional wound batteries still requires a large volume because each battery includes a single winding held inside of an individual conductive casing and/or pressure vessel. For example, as shown in FIG. 15, in a conventional battery, a winding W may be placed within a cup-shaped conductive casing A (commonly called a can). The open end of the casing A is covered by an end plate B attached to the casing by some sort of sealing member C. The sealing member C keeps the electrolyte within the cup-shaped casing A and electrically isolates the plate B from the casing A. A plastic disc D keeps the top of the winding W from electrically contacting the bent in neck of the casing, which could cause shorting of the battery. An electrically conductive tab E extending through a hole in the disc D is welded to one of the winding's electrodes and to the end plate B. A second tab F is welded to the other electrode and the bottom of the can A. A pressure relief valve G is provided in the end plate B to relieve excess pressure during recharging. The valve G may include a spring H which urges a stopper I against a hole J in the bottom of the plate B. Excess pressure causes gases to force the stopper upward against the spring H thereby allowing gases to escape through the valve G. Thus, providing a plurality of batteries having separate casings A, end plates B, sealing members C, and valves G thus takes up space, adds weight to the energy storage device and increases the chance of electrolyte leakage due to seal failure.
Fabrication of a storage device comprising a plurality of separate batteries is complicated and results in a great number of potential points of failure within the device. The casings must contain gases produced within the windings as the batteries are recharged, and thus must act as individual pressure vessels, each having separate seals and pressure relief valves. If a plurality of separate pressure vessels are used, failure of one seal could render the entire group of batteries inoperable. As windings are made more massive, the forces produced during recharging become higher increasing the possibility of failure of the seals.
In light of the foregoing, there is a need for a battery and battery operated vehicle, and a method and apparatus for manufacturing such a battery which overcome these disadvantages.